The term “impact compactor” typically refers to a soil compaction device which includes one/two impact compactor drum/s of non-round shape which, when towed/driven over a soil surface, produces a series of periodic impact blows on the soil surface. These periodic blows compact the soil which results in packing and orientating the soil into a more dense and effective particle arrangement, which reduces air voids and prevents further densification and shear failure of the soil.
The impact compactor drum/s of the impact compactor each has a series of spaced apart salient points on its periphery with each such salient point followed by a compacting face. As the impact compactor is towed over the soil surface, for instance by means of a tractor, the compactor drum rises up on each salient point and then falls forwardly and downwardly as it passes over that point, with the result that the following compacting face applies an impact blow to the soil surface. The function of the impact compactor drum is therefore to store potential energy as it rises up on each salient point and then to deliver this energy as an impact blow.
Conventional impact compactors (see FIG. 10) are generally larger than vibratory rollers (see FIG. 9) and they only operate in one direction of travel with the impact compactor generally being towed by a large four-wheel drive tractor. In order to make a compaction pass in the reverse direction, such a conventional impact compactor has to be turned around through 180 degrees. In order to do so the impact compactor has to reduce speed and because of its large turning radius, make a wide turn which is frequently wider than the width of the working surface of the construction site that requires compaction. This process places severe strain on the impact compactor and its tractor; can disturb existing prepared surfaces; and wastes valuable time. When working on a typical construction site, for example a roadway, as much as 50% of the impact compactor's time may be wasted in turning movements at each end of the compaction run. Furthermore, on some construction sites, it may be impossible to turn the machine where the site is too narrow or where there is a high embankment with a narrow working surface. This limitation has historically eliminated the use of conventional uni-directional impact compactors on such sites.
In the early impact compactor design process, in order to eliminate jerk loads between the impact compactor drum and the tow tractor, it was established that a spring type of connection was needed to operate between the tow tractor and the impact compacting drum. The spring needed to operate in such a manner that after the drum axle reaches its highest point of lift above the ground surface, the axle centre should not be obstructed in its forward and downward movement by which compaction of the soil is achieved. In early machines built with coil springs or leaf springs connecting the impact compactor to the tow tractor, the traction force varied according to formula F=Rd where R is spring stiffness in Newton/mm and d the deflection of the spring. Since the traction force varies as the value of d varies with the motion of the impact compactor, there was an unacceptably large cyclical jerk on the tractor. This problem was overcome by providing a hydraulic system where a ram was activated in and out of a hydraulic cylinder, to drive fluid into an accumulator of large capacity relative to the amount of fluid displaced by the movement of the ram in and out of the cylinder. Through this system, the change in volume of the pressurising gas in the hydraulic accumulator varied only slightly over the in and out cycle of the ram. By using this system, impact compactors have been made acceptable to the construction industry because tractors are not damaged by fluctuating push and pull forces and tractor operators have a relatively smooth ride (see FIG. 10).
Although the above system works for impact compactors that can operate only in a forward direction, it cannot practically be used for an impact compactor that needs to operate in both the forward and reverse directions of travel.
Attempts have been made to design a uni-directional self-propelled dual drum impact compactor. Since these self-propelled impact compactors typically have only two driven wheels on the tractor (i.e. the front part of the self-propelled impact compactor which drives the compactor), this type of impact compactor has insufficient traction to cope with most surface conditions on construction sites, especially when travelling over non-cohesive (sandy) or over-wet soils. This presents a problem because impact compactors are frequently required to work on these types of soils.
Typically 80% of the weight of a dual drum compactor is in the drums. When the drums are lifted off the ground, the weight of the drums is supported by the pneumatic tyres that are located between the drums. On an even and level surface, the centre of gravity of the impact compactor will be above the centre between the tyres and the impact compactor should be stable. If the impact compactor travels over an uneven surface or a surface that is not level, the centre of gravity will shift and could cause the impact compactor to become unstable. The instability caused by the large proportion of weight of the drums being on the outside of the tyres results in the risk that should the centre of gravity shift too much to one side, the impact compactor may roll excessively relative to the tractor, causing the compactor drums to fall sideways and strike the ground surface. This has previously resulted in damage to the compactor and the road surface, as well as the tractor. Any deformation of the pneumatic tyres, due to the properties of the tyres allowing them to change shape when striking an object for example, would also add to the instability problem.
Conventional impact rollers may be fitted with a blade which functions to move soil displaced by the action of the impact compactor drum/s, into the indentations created by the drum blows on the soil surface. Such blades are, however, designed to work only in a forward direction in which the impact compactor operates.
An impact compactor of conventional design delivers each blow over a large area of contact with the ground surface. For that reason it does not operate effectively on tightly cemented soils, such as laterite and calcrete, nor is it particularly effective in compacting previously traffic-compacted un-surfaced gravel roads or in rehabilitating existing paved roads.
It is an aim of the present invention to provide means which will at least alleviate some of the above identified problems.